Next generation mobile communications such as the Universal Mobile Telecommunications System (UMTS) and the UMTS Long Term Evolution (LTE) aim to offer improved services to the user compared to the existing systems. These systems are expected to offer high data rate services for the processing and transmission of a wide range of information, such as voice, video and IP multimedia data.
LTE is a packet-based technology for the delivery of high speed data services with increased data rates for the users. Compared to UMTS and previous generations of mobile communication systems, LTE will also offer reduced delays, increased cell edge coverage, reduced cost per bit, flexible spectrum usage and multi-radio access technology mobility.
LTE has been designed to give peak data rates in the downlink (DL) (communication from a base station (BS) to a subscriber station or user equipment (UE) of >100 Mbps whilst in the uplink (UL) (communication from the UE to the BS) >50 Mbps.
The basic system architecture in LTE is illustrated in FIG. 1. As can be seen, each UE 12 connects over a wireless link via a Uu interface to an eNB 11, which defines one or a number of cells for wireless communication. There is a network of eNBs referred to as the “eUTRAN” 10.
Each eNB 11 in turn is connected by a (usually) wired link using an interface called S1 to higher-level or “core network” entities, including a Serving Gateway (S-GW 22), and a Mobility Management Entity (MME 21) for managing the system and sending control signalling to other nodes, particularly eNBs, in the network. In addition, a PDN [Packet Data Network] Gateway (P-GW 23, FIG. 2) is present, separately or combined with the S-GW 22, to exchange data packets with any packet data network including the Internet. The wired links to the core network EPC 20 (where EPC stands for Evolved Packet Core) are referred to as “back haul” and will employ Internet Protocol (IP), over existing broadband infrastructure in homes and offices.
In FIG. 1, the S1 interface is labelled S1-U, the suffix—U denoting the user plane employed by the eNBs 11 for communicating user data to and from the S-GW 22. The S-GW is responsible for packet forwarding of user data on the downlink to the UE 12 and on the uplink. The S-GW 22 provides a “mobility anchor” for the user plane during handovers of a UE 12 from one eNB 11 to another. It also manages and stores UE “contexts” which are the details of active connections with UEs including so-called bearers (see below).
In parallel to this, there is an interface S1-MME (sometimes called S1-C) via which the eNBs 11 exchange control messages with the MME 21. The main function of the MME 21, as its name suggests, is to manage mobility of the UEs 12, and it is a signalling-only entity; in other words, user data packets do not pass through the MME. The MME 21 is also responsible for controlling security (including authenticating users), and for EPS bearer control (see below). In practice, there may be several MMEs forming a MME “pool”. One eNB can have several S1-MME interfaces towards several MMEs.
In addition, as shown in FIG. 1, the eNBs 11 communicate among themselves by a (usually) wireless link, using an interface called X2 for mutual co-ordination, for example when handing over a UE 12 from one cell to another. There is only one X2 interface between two eNBs.
In the above configuration, communications among eNBs can be regarded as communications among peers (network nodes at the same hierarchical level) with the MME constituting a higher level entity in the system.
A further aspect of the control signalling relates to so-called Operations, Administration, Maintenance (OAM). By exchanging OAM signalling with an OAM server (not shown), the RNs and eNBs can co-ordinate their actions, making the network self-organizing at least to some extent.
Separating the management functions from the handling of user data traffic—in other words, splitting the user plane from the control plane—allows the respective hardware resources (MMEs, S-GW) to be scaled independently as required for the number of users and volume of traffic. A multi-vendor arrangement is also possible, in which service providers deploy their own core networks 20 but use the same eUTRAN 10.
The concept of “bearers” is important for achieving quality-of-service (QoS) in a packet-based network such as LTE. In general, a “bearer” can be thought of as an information transmission path of defined capacity, delay and bit error rate, etc. so as to enable a given service or control function to be provided. Various types or levels of bearer can be established, the radio part being set up using radio resource control or RRC.
FIG. 2 shows an EPS Bearer Service Architecture proposed for LTE. The left side of the Figure represents the eUTRAN 10 with the EPC 20 occupying the middle part of the Figure. At the right-hand side, outside the LTE system as such, there is the Internet 24. The vertical bars represent the main entities in the user plane, from the UE 12 to eNB 11 through to S-GW 22 and P-GW 23, terminating in a peer entity (such as an Internet web server 25) connected to the P-GW 23. To provide an end-to-end service 40 between the UE 12 and Peer Entity 25 (as indicated by the upper horizontal band in the figure), the system sets up “bearers” as shown. An EPS Bearer 41 represents the entire connection within the LTE system; it constitutes a QoS flow for a particular service. The connection continues outside the LTE system via an External Bearer 42.
The EPS Bearer 41 is made up, in turn, of a radio bearer 51 over the link between the UE 12 and eNB 11, and an S1 Bearer 52 between the eNB 11 and S-GW 22. A further Bearer (S5/S8 Bearer 53) is set up between the S-GW 22 and P-GW 23. Each Bearer can be regarded as a “tunnel” in a given protocol layer for transport of packets, connecting the end points for the duration of a particular service or “session”, e.g. voice call or download. Thus, the radio bearer 51 transports the packets of the higher-layer EPS Bearer 41 between the UE 12 and eNB 11, and the S1 Bearer 52 transports the packets of the EPS Bearer 41 between the eNB 11 and S-GW 22. Bearer control through RRC, mentioned previously, includes the setting up of bearers for a particular session so as to ensure sufficient QoS, taking into account the resource situation in the E-UTRAN 10 and existing sessions already in progress. It also involves the modification and release of radio bearers.
Bearers are also defined in the signalling plane. As currently proposed, LTE employs the known SCTP protocol (see below) for transmitting signalling messages between an eNB 11 and MME 21, or between two eNBs. To be precise, SCTP is used in the transport layer of an S1 or X2 signalling bearer as the case may be. This requires a so-called SCTP association setup to be performed between the eNB and MME, or between the two eNBs.
LTE-Advanced (LTE-A), currently being standardised, will further improve the LTE system, using new techniques to improve the performance over existing LTE systems, providing higher data rates (up to 1 Gbps DL, 500 Mps UL) and improvements to cell edge coverage. Support for relay nodes (RNs) is part of this effort.
Currently being considered are so-called “Type 1” RNs. These are relays which connect to the network “in-band”, in other words using the same frequency band(s) as UEs, and which set up their own cells such that each RN appears to a UE to be like an eNB. Thus, a UE attached to a given RN communicates with the RN via the Uu interface, just as it would with an eNB. In other words the RN acts like an eNB to a great extent. The “in-band” radio interface between the RNs and eNBs is called Un, to distinguish it from Uu.
Transmission of messages between nodes in a radio network, such as between the RNs and eNBs, involves the use of multi-layer protocol stacks. On the transmission side, starting from the top of the stack at an application layer, each layer in the protocol stack processes units of data in some way, usually adding a header to the data unit before passing it down to the next lower layer or sub-layer. The headers include fields identifying the operations performed at that protocol layer. On the reception side, each layer decodes the header inserted in the corresponding transmission-side layer to allow reconstruction of a data unit, which is then passed up to the next higher layer.
Via the two radio interfaces (Uu and Un), user data traffic is transported by the User-Plane, consisting of Packet Data Convergence Protocol (PDCP), Radio Link Control (RLC), Medium Access Control (MAC) and PHYsical (PHY) protocol layers. FIG. 3 shows the relationship between the protocol layers for the LTE user plane, labelled (1), and the control plane (2).
A particular concern in the present invention is control signalling via the S1 and X2 interfaces, so this will be explained in more detail with reference to FIGS. 4 and 5.
S1 Control Plane
The S1 control plane interface (S1-MME) is defined on the link between the eNB and the MME. The control plane protocol stack of the S1 interface is shown in FIG. 4. The transport network layer 32 is built on IP transport, similarly to the user plane but for the reliable transport of signalling messages, SCTP 31 is added on top of IP. Lower layers of the protocol stack include a data link layer 33 and the physical layer 34. The data link layer 33, in turn, comprises a Packet Data Convergence Protocol (PDCP) sub-layer, a Radio Link Control (RLC) sub-layer, and a Media Access Control (MAC) sub-layer. The MAC layer forms S1 signalling messages or other data into data units (MAC PDUs) suitable for transmission over the radio network. These are received by the physical layer or PHY, which provides the link from each network node to the radio resources of the network. The application layer signalling protocol is referred to as S1-AP 30 (S1 Application Protocol). There are generally two types of S1-AP message: (a) UE-dedicated messages (specific to an individual UE 12) and (b) messages for common procedures.
As already mentioned, a RN acts like an eNB at least from the viewpoint of a UE. Therefore, in addition to the S1-MME interface between the eNB and MME, a “logical” S1 interface is defined over the whole path between the MME and RN.
X2 Control Plane
The X2 control plane interface (X2-CP) is defined on the link between two neighbour eNBs 11. The control plane protocol stack of the X2 interface is shown on FIG. 5. The transport network layer is built on SCTP 36 on top of IP 36, with data link layer 38 and physical layer 39 as before. The application layer signalling protocol is referred to as X2-AP 35 (X2 Application Protocol).
Again, a logical X2 interface exists from an eNB to an RN, when RNs are employed.
SCTP
The SCTP layer 31 or 36 in FIGS. 4 and 5 provides the guaranteed delivery of application layer messages through the SCTP association(s) established between two nodes. Application layer protocols submit their data to be transmitted in messages to the SCTP transport layer. SCTP places messages and control information into separate chunks (data chunks and control chunks), each identified by a chunk header. A message can be fragmented over a number of data chunks, but each data chunk contains data from only one user message. SCTP chunks are bundled into SCTP packets. The SCTP packet, which is submitted to the IP layer, consists of a packet header, SCTP control chunks when necessary, followed by SCTP data chunks when available.
SCTP allows for delivery of chunks within independent streams, to avoid unnecessary head-of-line blocking. Head-of-line blocking (HOL) is a phenomenon that appears in buffered telecommunication network switches. A switch is usually made of buffered input ports, a switch fabric and buffered output ports. Because of the FIFO nature of the input buffers and switch design, the switch fabric can only switch the packets at the head of the buffer per cycle. HOL arises when packets arriving at different input ports are destined for the same output port. If the HOL packet of a certain buffer at the input cannot be switched to an output port because of contention, the rest of the packets in that buffer are blocked by that Head-of-Line packet, even if there is no contention at the destination output ports for those packets. The phenomenon may have severe performance-degrading effects in input-buffered systems.
Each message sent over an SCTP association is assigned to a particular stream. All data within a stream is delivered in order with respect to other data in that stream. Data in different streams have no order constraints. SCTP's resulting parallel ordered streams provide a specific instance of ‘partial ordered’ delivery. It is SCTP's multi-streaming service that circumvents the head-of-line blocking problem discussed above.
In LTE, a single SCTP association per S1-MME interface instance is used with one pair of stream identifiers for S1-MME common procedures. At least one pair of stream identifiers should be used for S1-MME dedicated procedures. MME communication context identifiers that are assigned by the MME for S1-MME dedicated procedures, and eNB communication context identifiers that are assigned by the eNB for S1-MME dedicated procedures are used to distinguish UE specific S1-MME signalling transport bearers. The communication context identifiers are conveyed in the respective S1-AP messages.
Similarly, in the case of X2 interface, a single SCTP association per X2 interface instance is used, with a single pair of stream identifiers reserved for the sole use of X2-AP elementary procedures that utilize non UE-associated signalling. At least one pair of stream identifiers is reserved for the sole use of X2-AP elementary procedures that utilize UE-associated signalling. However a few pairs (i.e. more than one) should be reserved.
These X2-AP elementary procedures using non UE-associated signalling are also known as common procedures. These procedures include Reset, X2 Setup, eNB Configuration Update, Resource Status Reporting Initiation, Mobility Settings Change, Load Indication, Resource Status Reporting, Error Indication, Radio Link Failure Indication, Handover Report and Cell Activation. These procedures are not associated with any specific UE.
In addition, there is the signalling associated with Operations, Administration, Maintenance (OAM) as already mentioned.
There is only limited provision for data bearers (SRBs and DRBs) over the Un interface. Consequently there is the problem that in the RN←→DeNB Un interface, all the S1-AP, X2-AP and/or OAM messages with different QoS requirements (for example different S1-AP/X2-AP messages that have different time criticalities) have to be delivered in a limited number of DRBs or SRBs. This problem is fundamentally a Head of Line (HOL) blocking issue that arises when packets arriving at different input ports are destined for the same output port.